1. Field of the Invention
The present invention relates generally to the manufacture of layered webs comprised of more than one component material and, more particularly, but not by way of limitation, to the manufacture of webs of magnetic recording material comprised of a dielectric film coated with a binder containing a magnetic material.
2. Brief Discussion of the Prior Art
Consumer and industrial goods include a wide variety of products which have the general form of a composite built up of thin layers of two different materials. For example, magnetic recording tape commonly consists of a thin strip of polyester coated on both sides with a layer of epoxy containing ferric oxide. These composites can be manufactured by a variety of techniques; for example, by various coating techniques in which a plasticized layer of one material is deposited on a moving web of the other material. Similarly, the product can, in some cases, be manufactured by simultaneously extruding the two materials in the form of two webs in contact. In any event, it is common practice to form very wide webs of these composites which are later cut to a desired shape or width to minimize the cost of manufacturing the product.
For the ultimate product to serve its intended purpose, the thicknesses of the layers must meet standards which are often embodied in a requirement that the specific weight; that is, the weight per unit area, of each of the two components of the web is to lie within preselected limits. In order to meet these standards, the specific weights are measured during the manufacture of the web so that the machinery used in the manufacture can be adjusted to maintain the preselected specific weights for the components of the web. A device that is commonly used in the measurement of the specific weight of a web is a radiation sensor that includes a source of radiation that can be placed on one side of the web and a radiation detector that can be placed on the other side of the web to provide a signal that is a measure of the attenuation of a beam emitted by the source as the beam passes through the web. Such a signal can be converted to a specific weight signal that can be transmitted to a display or a control device so that the machinery which produces the web can be either manually or automatically adjusted to cause the web to meet the required standards.
In the past, the monitoring of specific weights of the components of a composite web has often been carried out by measuring the absorption of radiation by a web formed of one of the components followed by a similar measurement on the web after the second component has been added to the web. This approach has several disadvantages. For economy of manufacture, the web width will often be very large so that the web must be scanned by moving the radiation sensor back and forth across the width of the web in order to monitor the specific weights of the web components throughout the width. The cost of the machinery to effect this scanning is by no means inconsequential. Moreover, separate scanning of the one-component web and the finished product necessarily requires that the two sensors used for this purpose make measurements on widely spaced portions of the web. This spacing can result in appreciable lengths of non-standard web being produced before a divergence from tolerances can be detected and corrected. Because of these problems, it would be preferred to have only one device, including two sensors located together immediately downstream of the machinery which completes the web manufacturing process, to make the specific weight determinations.
However, the inclusion of several sensors in one scanning head that traverses only the finished product also presents problems. In general, both sensors will respond to both materials of which the web is comprised so that calibration of an assembly of sensors to yield both specific weights can be difficult and may require the acceptance of approximations that detract from the ability of the assembly to accurately measure the specific weights of the two web components. Moreover, where several sensors are mounted on one scanning head, cross talk between the sensors can become a problem. That is, the detector of one sensor responds to radiation produced by the source of the other sensor so that the signal produced by such detector does not provide an accurate measure of the effect of the web on the radiation beam that the detector is designed to receive. Moreover, the mechanism by means of which the cross talk occurs can involve the interaction of the radiation beams of the sensors with the web itself so that calibration of the sensors to compensate for cross talk can become a difficult proposition.